Spin cast self-assembled monolayer field effect transistors

doi:10.1016/j.orgel.2011.11.025

Organic Electronics

Volume 13, Issue 3, March 2012, Pages 464-468

https://doi.org/10.1016/j.orgel.2011.11.025 Get rights and content

Abstract

Top-contact self-assembled monolayer field-effect transistors (SAMFETs) were fabricated through both spin-coating and solution assembly of a semiconducting phosphonic acid-based molecule (11-(5⁗-butyl-[2,2′;5′,2″;5″,2‴;5‴,2⁗]quinquethiophen-5-yl)undecylphosphonic acid) (BQT-PA). The field-effect mobilities of both spin-cast and solution assembled SAMFETs were 1.1–8.0 × 10⁻⁶ cm² V⁻¹ s⁻¹ for a wide range of channel lengths (between 12 and 80 μm). The molecular monolayers were characterized by atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that the BQT-PA monolayer films exhibit dense surface coverage, bidentate binding, and tilt angles of ∼32° and ∼44° for the thiophene rings and alkyl chain, respectively. These results indicate that rapid throughput of fabricating SAMFETs is possible even by spin-coating.

Graphical abstract

Highlights

► A simple approach to fabricate SAMFETs through a rapid spin-coating process. ► Charge mobilities of 1.1–8.0 × 10⁻⁶ cm² V⁻¹ s⁻¹ for channel lengths of 12–80 μm. ► In-depth monolayer characterization by NEXAFS, ATR-FTIR, and AFM.

Introduction

In recent years, organic field effect transistors (OFETs) have become the subject of intense research due to the appeal of inexpensive, solution processed, and mechanically flexible electronic devices [1]. Possible applications include radio frequency identification tags, smart cards, and drivers for active matrix displays [2]. Solution processing of organic molecules allows for the construction of various layers of device architecture by high-throughput and inexpensive means. Frequently studied is the use of π-conjugated organic molecules to form semiconducting films/layers [3].

The development of π-conjugated self-assembled monolayer (SAM) molecules has resulted in a new type of OFET; the self-assembled monolayer field effect transistor (SAMFET). Functionally, SAMFETs are OFETs in which the organic semiconductor is a single layer of well-packed π-conjugated molecules capable of acting as a charge-transporting channel. Fabrication of SAMFET devices has been attempted by several groups, however, most devices required channel lengths of submicrometer to ensure a gate voltage dependence of the source to drain channel current apparently due to limited lateral interconnection of the semiconducting SAM [4], [5], [6], [7]. Recently, SAMFETs with long channel lengths up to 40 μm have been demonstrated by Smits et al., in which a SAM was formed on a silica gate dielectric via a greater than 15 h immersion phase assembly [8]. Performance of these SAMFETs was comparable to that of OFETs constructed with a three-dimensional bulk film.

In this paper, we demonstrate rapidly processed SAMFETs achieved through spin-coating of a phosphonic acid-based molecule 11-(5⁗-butyl-[2,2′;5′,2″;5″,2‴;5‴,2⁗]quinquethiophen-5-yl)undecylphosphonic acid (BQT-PA). The resulting spin-cast SAMs show uniform density, and well-ordered monolayer coverage comparable to those observed by conventional immersion phase solution assembly. The top-contact SAMFETs processed by spin-coating and immersion assembly show identical electronic performance.

Section snippets

BQT-PA synthesis

BQT-PA was synthesized by Pd-catalyzed coupling between diethyl 11-(5′-bromo[2,2′]bithiophen-5-yl)undecylphosphonate and (5″-butyl[2,2′;5′,2″]terthiophen-5-yl)trimethyltin at 90 °C in toluene to afford diethyl BQT-phosphonate. By reacting with bromotrimethylsilane, the diethyl BQT-phosphonate was converted into ditrimethylsilyl BQT-phosphonate and then hydrolyzed to afford the target molecule BQT-PA.

SAMFET device fabrication

The architecture of the SAMFET device studied in this work is presented in Fig. 1. Heavily p

Results and discussion

As it has been demonstrated that charge transport in OFETs primarily occurs in the first few nanometers of the channel closest to the dielectric material, devices utilizing a single molecular layer for charge transport are attainable [9]. This ultra-thin semiconductor architecture has been shown to be highly desirable from the standpoint of chemical sensing, as slight electrochemical changes in the active layer result in measurable shifts in device performance [10]. One such method of obtaining

Conclusion

In conclusion, we have demonstrated a simple and generally applicable approach to fabricate SAMFETs through a rapid spin-coating process. Charge mobilities of 1.1–8.0 × 10⁻⁶ cm² V⁻¹ s⁻¹ were achieved for a wide range of channel lengths (from 12 to 80 μm). Characterization of the BQT-PA SAM was performed by AFM, ATR-FTIR, and NEXAFS, showing a densely packed monolayer with tilt angles of ∼32° and ∼44° for the thiophene rings and alkyl chains, respectively. This work is representative of major

Acknowledgements

This work is supported by the NSF-STC program under DMR-0120967, the AFOSR program under FA9550-09-1-0426. T. Weidner thanks the Deutsche Forschungsgemeinschaft for a research fellowship, and A.K.-Y. Jen thanks the World Class University (WCU) program through the National Research Foundation of Korea under the Ministry of Education, Science and Technology (R31-10035). Part of this work was conducted at the University of Washington NanoTech User Facility, a member of the NSF National

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LetterSpin cast self-assembled monolayer field effect transistors

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

BQT-PA synthesis

SAMFET device fabrication

Results and discussion

Conclusion

Acknowledgements

J. Chem. Phys.

J. Phys. Chem. C

Chem. Rev.

Angew. Chem. Int. Ed.

Adv. Mater.

Chem. Rev.

Adv. Funct. Mater.

Chem. Mater.

Adv. Mater.

Angew. Chem. Int. Ed.

J. Am. Chem. Soc.

Adv. Funct. Mater.

Nano Lett.

Letter
Spin cast self-assembled monolayer field effect transistors